Hitherto, as a power supply of an electronic equipment such as note type personal computer, mobile telephone, camera integrated type VTR (Video Tape Recorder) and/or PDA (Personal Digital Assistants), etc., a light secondary battery having high energy density is required. As the secondary battery having high energy density of this kind, there is lithium ion secondary battery having energy density larger than that of aqueous electrolyte battery, e.g., lead battery, nickel-cadmium battery and/or nickel-hydrogen battery, etc.
The lithium ion secondary battery includes, e.g., a battery element having cathode and anode, an armored can which is a cylindrical vessel having bottom portion for accommodating the battery element therewithin, and which is electrically connected to the anode so that the armored can serves as anode terminal, and a cover body adapted for closing the opening portion of the armored can and electrically connected to the cathode so that the cover body serves as an external cathode terminal. In the lithium ion secondary battery, as the result of the fact that cover body is press-fitted into the opening portion of the armored can through gasket thereafter to caulk the opening portion of the armored can so that the cover body is fixed to close the opening portion of the armored can, the battery element is hermetically sealed into the armored can. For this reason, in the lithium ion secondary battery, the armored can of the external anode terminal and the cover body of the external cathode terminal are placed in insulating state by gasket.
In the case where the lithium ion secondary battery of such a configuration is used as a power supply of the above-described electronic equipments, the lithium ion secondary battery is mounted at the electronic equipment in the state of battery pack. As the battery pack, there is a battery pack described in the Japanese Patent Application Laid Open No. 2002-343320 publication.
In the battery pack 100, as shown in FIG. 1, e.g., two lithium ion secondary batteryies 101 are accommodated within a pair of accommodating cases 103 in the state connected to a circuit wiring board 102 for performing over-charge protection, over-discharge protection and/or charge/discharge control, etc. with respect to the battery. In concrete terms, the lithium ion secondary batteries 101 are connected to the circuit wiring board 102 in series state through belt-shaped lead terminals 104 consisting of conductive metal, etc., e.g., nickel, iron or stainless steel, etc. At this time, in the battery pack 100, e.g., connection between the armored can 105 serving as external terminal and/or the cover body 106 in the lithium ion secondary battery 101 and the lead terminals 104 is performed by using the resistance welding method.
As shown in FIG. 2, this resistance welding method is a method of welding the lead terminal 104 and the cover body 106 by making use of heat by electric resistance produced between the lead terminal 104 and the cover body 106 in allowing current of about 1200 A to flow from one side of a pair of electrode rods 107, 108 disposed on the principal surface of the lead terminals 104 to the other side electrode rods thereof in the state where, e.g., lead terminal 104 is caused to be in contact with the cover body 106, etc.
In the case of welding lead terminals 104 with respect to the armored can 105 and/or the cover body 106 by such a method, it is necessary to thin thickness of the lead terminal 104 to a certain degree. In concrete terms, if the thickness of the lead terminal 104 consisting of conductive metal, etc. e.g., nickel or iron, etc. is not caused to be equal to about 0.15 mm, it becomes difficult to perform welding in which welding reliability has been enhanced.
The reason thereof is as follows. As shown in FIG. 3, in the case where, e.g., lead terminal 104 having thickness of about 0.2 mm is used, because the thickness of the lead terminal 104 is too thick, a current flowing in the lead terminal 104, so called reactive current at a path indicated by arrow X in FIG. 3 would take place at a portion of current flowing in the electrode rod 108 from the electrode rod 107 in performing resistance welding.
For this reason, in the welding between the lead terminal 104 and the armored can 105 or the cover body 106, current at a path indicated by arrow Y in FIG. 3, i.e., current flowing in a thickness direction of the lead terminal 104 to reach the armored can 105 or the cover body 106, so called active current is decreased so that calorific value by electric resistance of the lead terminal 104 is reduced. Thus, in welding between the lead terminal 104 and the armored can 105 or the cover body 106, there are instances where welding blocks, so-called welding nuggets 109 formed as the result of the fact that the lead terminal 104 and the armored can 105 or the cover body 106 are molten with each other by heat of electric resistance by active current of the lead terminal 104 is reduced so that welding strength may be weakened.
Particularly, in the case where lead terminal 104 is formed by conductive metal, etc. containing, e.g., copper, silver or aluminum, etc. having electric resistance smaller than that of the armored can 105 or the cover body 106, ratio of reactive current becomes large among current flowing in the electrode rod 108 from the electrode rod 107 to the electrode rod 108. As a result, welding strength between the lead terminal 104 and the armored can 105 or the cover body 106 is further weakened.
In such lithium ion secondary battery 101, in the case where, e.g., the battery pack 100 is erroneously fallen down in detaching it from electronic equipment, etc. so that the lithium ion secondary battery 101 undergoes impact, etc., because welding strength between the lead terminal 104 and the armored can 105 or the cover body 106 is weak, there are instances where connection between the lead terminal 104 and the armored can 105 or the cover body 106 is broken to allow the battery pack 100 to be brough into unusable (unavailable) state.
Moreover, in the battery pack 100, in the case where the lead terminal 104 is thinned for the purpose of enhancing welding reliability with respect to the armored can 105 and/or the cover body 106, electric resistance of current flowing in a length direction in the lead terminal 104 in charging/discharging the lithium ion secondary battery 101 would become large. For this reason, in the battery pack 100, there are instances where the lead terminal 104 having large electric resistance may be heated by current of charge/discharge operation so that the lithium ion secondary battery 101 is deteriorated by such heat. Namely, in the battery pack 100, it becomes difficult to use the lithium ion secondary battery 101 as a power supply of recent electronic equipments caused to further have high function and high performance so that there is required large current discharge in which discharge operation is performed at current of about 1 C to 2 C per battery.
Further, in such lead terminal, because electric resistance is large, voltage drop takes place in the lithium ion secondary battery 101 at the time of charge or discharge operation. As a result, power loss takes place, leading to lowering of energy utilization efficiency.